The nature and severity of the attention deficits and behavior problems associated with Attention Deficit Hyperactivity Disorder (ADHD) in children ages 8-16 with Mental Retardation (MR) will be explored. This proposed project will be a continuation of an NIMH Small Grant and will use the attentional measures developed in that Small Grant to compare MR children with ADHD ("MR/ADHD") to MR children without ADHD ("MR/non-ADHD") in order to ascertain which components of attention (sustained attention, impulsivity, selective attention, immediate memory) are problematic in MR/ADHD children. This difference between MR/ADHD children and MR/non-ADHD will be compared to the difference between mental age matched ADHD and non-ADHD children of normal intelligence to explore the relationship between ADHD and MR with regard to attention. Since it is hypothesized that MR and ADHD are not entirely unrelated to each other, the pattern of attention deficits associated with ADHD in MR children is predicted to be different from the pattern of attentional deficits noted in ADHD children of normal intelligence. Finally, stimulant treatment of ADHD will be assessed using placebo, 0.15 mg/kg, 0.3 mg/kg, and 0.6 mg/kg of methylphenidate in a double-blind, placebo-controlled, crossover design with the MR/ADHD children and the mental age matched ADHD children to determine optimal dosages of this medication in regard to attentional processes and behavior for both of these two groups.